The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for a chemical mechanical polishing system.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly more non-planar. This non-planar outer surface presents a problem for the integrated circuit manufacturer. If the outer surface of the substrate is non-planar, then a photoresist layer placed thereon is also non-planar. A photoresist layer is typically patterned by a photolithographic apparatus that focuses a light image onto the photoresist. If the outer surface of the substrate is sufficiently non-planar, then the maximum height difference between the peaks and valleys of the outer surface may exceed the depth of focus of the imaging apparatus, and it will be impossible to properly focus the light image onto the outer substrate surface.
It may be prohibitively expensive to design new photolithographic devices having an improved depth of focus. In addition, as the feature size used in integrated circuits becomes smaller, shorter wavelengths of light must be used, resulting in further reduction of the available depth of focus. Therefore, there is a need to periodically planarize the substrate surface to provide a substantially planar layer surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted to a carrier or polishing head. The exposed surface of the substrate is then placed against a rotating polishing pad. The carrier provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. In addition, the carrier may rotate to provide additional motion between the substrate and polishing pad. A polishing slurry, including an abrasive and at least one chemically-reactive agent, is distributed over the polishing pad to provide an abrasive chemical solution at the interface between the pad and substrate. A CMP process is fairly complex, and differs from simple wet sanding. In a CMP process the reactive agent in the slurry reacts with the outer surface of the substrate to form reactive sites. The interaction of the polishing pad and abrasive particles with the reactive sites results in polishing.
An effective CMP process has a high polishing rate and generates a substrate surface which is finished (lacks small-scale roughness) and flat (lacks large-scale topography). The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad. Because inadequate flatness and finish can create defective substrates, the selection of a polishing pad and slurry combination is usually dictated by the required finish and flatness. Given these constraints, the polishing rate sets the maximum throughput of the polishing apparatus.
The polishing rate depends upon the force pressing the substrate against the pad. Specifically, the greater this force, the higher the polishing rate. If the carrier head applies a non-uniform load, i.e., if the carrier applies more force to one region of the substrate than to another, then the high pressure regions will be polished faster than the lower pressure regions. Therefore, a non-uniform load may result in non-uniform polishing of the substrate.
An additional consideration in the production of integrated circuits is process and product stability. To achieve a high yield, i.e., a low defect rate, each successive substrate should be polished under substantially similar conditions. Each substrate should be polished by approximately the same amount so that each integrated circuit is substantially identical.
In view of the foregoing, there is a need for a chemical mechanical polishing apparatus which optimizes polishing throughput, while providing the desired flatness and finish. Specifically, the chemical mechanical polishing apparatus should have a carrier head which applies a substantially uniform load to the substrate.
In one aspect, the invention is directed to an apparatus for use with a carrier head of a chemical mechanical polishing apparatus. A module has a recess, and a layer of conformable material is disposed in the recess to provide a mounting surface for a substrate. The module is detachably connected to the carrier head.
Implementations of the invention may include the following. The carrier head may have a backing fixture, and a loading mechanism may connect the backing fixture to the housing. The module may be mechanically or magnetically connected to the carrier head. The module may have a rim surrounding the recess, and the conformable material may be flush with the rim.
In another aspect, the invention is directed to a carrier head for positioning a substrate on a polishing surface in a chemical mechanical polishing apparatus. A base assembly has a recess, and a layer of conformable material is disposed in the recess to provide a mounting surface for a substrate. A retaining ring is connected to the mounting surface.
In another aspect, the carrier head has a base assembly, a layer of conformable material, and a shield ring which is connected to the base assembly and projects over a portion of the layer of conformable material.
Implementations of the invention may include the following. The retaining ring may be approximately the same thickness as the substrate. The shield ring may surround, but be thinner than, the retaining ring. An upper surface of the shield may be adjacent to the rim of the base assembly and be flush with the conformable material. The shield may be positioned to prevent the conformable material from extruding when the substrate is pressed against the polishing surface.
In another aspect, the carrier head has a base assembly, a layer of conformable material, and a chucking mechanism to attach the substrate to the mounting surface.
Implementations of the invention may include the following. The chucking mechanism may include a pump and a passageway through the layer of conformable material connecting the passageway to the mounting surface. The passageway may have a diameter such that it does not collapse if the pump applies suction to the passageway. The chucking mechanism includes an actuating mechanism, and a movable section of the base assembly may be connected to the actuating mechanism. The vertical motion of the movable section may form a pocket between a substrate and the layer of conformable material to suction the substrate to the mounting surface.
In another aspect, the invention is directed to a carrier head having a base assembly and a conformable material disposed in a recess of the base assembly. The conformable material has a durometer measurement selected to provide both elasticity and normal strain in response to an applied load.
Implementations of the invention include the following. The conformable material may have a durometer between about fifteen and twenty-five, such as about twenty-one. The conformable material may be substantially pure urethane. A sheet of non-adhesive material may be attached to the underside of the conformable material to provide the mounting surface.
Advantages of the invention include the following. The carrier head includes a conformable layer that applies a uniform load to the substrate. The conformable layer is chemically inert vis-a-vis the polishing process. The carrier head is also able to vacuum chuck the substrate to lift the substrate off the polishing pad.
Additional advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention may be realized by means of the instrumentalities and combinations particularly pointed out in the claims.